1. Field of the Invention
The invention relates to a method of generating slice images of an examination zone which is irradiated by X-rays from a plurality of radiation source positions in order to generate separate single projection images, for each single image there being stored image values which correspond to the absorption in its image points, slice images being generated in that slice image values are derived from the image values of the image points of the single images which are geometrically associated with a slice image point.
An image point is to be understood to mean hereinafter a finite, preferably square zone of a single image, whereas a slice image point is to be understood to mean a corresponding zone in the slice image.
2. Description of the Related Art
A method and a device of this kind are known, for example from U.S. Pat. No. 3,499,146. The single images are generated therein by means of an X-ray source which is successively moved to different radiation source positions. The radiation relief is picked up by an image converter, for example an image intensifier whose exit luminescent screen image is scanned by means of a television camera. The video signal thus generated is digitized. The digital data words produced correspond to the image values of the single image. They are stored in a preferably digital storage device. The slice image values for the various slice image points are derived from the image values of image points geometrically associated with the relevant slice image point, i.e. of those image points which are situated on the connecting line between the relevant slice image point and the various radiation source positions during the generating of the layer image.
It has been found that areas which are situated within the examination zones but outside the layer whose details are to be sharply reproduced can incur contrast reversal or pseudo-resolution in the layer image when the radiation source positions are situated on a straight line or on a circular arc about a horizontal axis.
It is known that such effects can also occur in conventional systems for linear tomography in which a layer image is formed by continuous shifting between a film and an X-ray source. It is known (see Harding et at. in Phys. Med. Biol., 1975, Vol. 20, No. 1, pp. 144-149) that these effects can be eliminated by making the X-ray tube current increase and decrease according to a triangle-shaped function during the exposure. According to an improved version of this method (Phys. Med. Biol., 1977, Vol. 22, No. 4, pp. 747-759), instead of the delta modulation a bell-shaped modulation curve is used, thus achieving a better compromise between suppression of undesirable effects and deterioration of blurring of details outside the layer.
Both methods necessitate an intervention in the X-ray generator feeding the X-ray source and require sufficiently fast control of the current during a layer exposure.